1. Field of the Invention
The present invention relates to exhaust gas filters for collecting fine particles which are contained in exhaust gases discharged from internal combustion engines, such as diesel engines, and apparatuses for treating exhaust gases with such filters.
2. Description of the Prior Art
Exhaust gases generally contain fine particles comprising, as a main ingredient, carbon, other than nitrogen oxides NO.sub.x, carbon monoxide CO, hydrogen carbides HC or the like. These fine particles per se not only cause air pollution but also deteriorate, as poison, catalytic activity of catalysts for purifying NO.sub.x, CO, HC or the like. Therefore, various exhaust gas filters for collecting those fine particles have so far been proposed.
Exhaust gas filters require characteristics, such as low pressure loss, high efficiency of collecting fine particles, high compressive strength, high thermal shock resistance or the like. Additionally, it is important that the exhaust gas filters can be regenerated with high efficiency, because the filters, since fine particles deposit thereon during filtration, require an intermittent regeneration by removing the deposits. If the regeneration efficiency is low, long use of the filter will result in increase of its pressure loss.
Japanese Patent Application Laid-open No. 03-47,507 discloses a technique for obtaining an excellent filter by superimposing a filter layer having an average pore diameter of 0.2-10 .mu.m on a filter substrate having an average pore diameter of 10-100 .mu.m and a ratio of the pore diameter in the position of 75 vol. % to that in the position of 25 vol. %, with respect to a cumulative pore distribution, of at least 1.3, which filter layer is fixed on the filter substrate in such a manner that the filter layer may block open-pores on the surface of the filter substrate.
As a process for regenerating filters, it has been known that collected fine particles are burnt up in situ on the filters by raising the temperature of the filters. Alternatively, there has also been known another conventional process wherein collected fine particles on filters are blown away by injecting blowback air into the filters counter to exhaust gas flow, and then the fine particles are burnt up. The latter process wherein the fine particles are blown away by blowback air has the advantage in that the life of the filters is generally extended, as compared with the former process wherein the fine particles are burnt up in situ on the filters.
However, the above conventional blowback process has posed a problem of an insufficient ability of regenerating filters during the blowback, resulting in increase of pressure losses with the lapse of time of collection operation, though it may partly depend on the properties of the filters. Alternatively, in the case where the filters are formed into a two-layer structure such as disclosed in Japanese Patent Application Laid-open No. 03-47,507, even with such a filter, a problem of increase in pressure loss has been posed, though it may depend upon the material that forms the filter layer.